JP7250800B2 - How to control chemical balance in pulp mill - Google Patents

Description

The present invention relates to a method of controlling chemical balance in a sulfate pulp mill that produces at least chlorine dioxide (ClO ₂ ) bleached pulp and has a chlorine dioxide plant.

Pulp mills' current chemical recovery arrangements primarily cover the recovery and reclamation of cooking chemicals, sodium hydroxide (NaOH) and sodium sulfide ( _Na2S ). Nevertheless, pulp mills produce significant amounts of side streams that can be harmful to the environment, but also contain significant amounts of valuable sodium and sulfur that are lost with these streams. I'm in.

These side streams are usually produced when the introduction of sulfur compounds from outside into the process exceeds the amount of sulfur required for digestion. This leads to excess sulfur in the pulp mill, which must be vented from the process. Excess sulfur is typically discharged into nearby water systems as sodium sulfate (Na ₂ SO ₄ ). This is the most acceptable method of returning the sulfur waste stream to nature. For example, most of the recovery boiler fly ash (80-90%) is sodium sulfate. Traditionally, fly ash discharged from chemical recovery cycles has been dissolved in water or condensate and the solution transported to wastewater treatment plants. Due to increasingly stringent environmental regulations, sodium sulfate emissions damage inland water systems and need to be significantly reduced.

Pulp mills use chlorine dioxide to bleach chemical pulp. There are several commercial chlorine dioxide production processes and are known per se. All processes use sodium chlorate (NaClO ₃ ) as raw material. This process also requires an acid and a reducing agent. Current typical processes include the R8 and R10 processes. The reducing agent is methanol (CH ₃ OH) and the acid is sulfuric acid (H ₂ SO ₄ ). Chlorine dioxide plants are therefore one of the major consumers of sulfuric acid and methanol in pulp mills. Additionally, these chemicals are brought in from outside the factory due to strict quality requirements.

Chlorine dioxide plants produce by-products that include sulfur and sodium. The R8 process produces the acid salt sodium sesquisulfate (Na ₃ H(SO ₄ ) ₂ ) and the R10 process produces sodium sulfate. They are often carried into chemical recovery cycles and dissolved in waste water. In the latter case, sulfates do not burden the plant's S/Na balance, but increase the plant's wastewater discharge. However, since a complete recovery cycle for these chemicals has not been achieved, the chemicals formed by their introduction into the process, primarily sulfur surpluses, must be vented to maintain the chemical balance of the process. be.

It is an object of the present invention to provide a method by which the introduction of chemicals into a pulp mill process can be significantly reduced or eliminated altogether, thereby minimizing the need to vent chemicals from the process. is.

The new process forms an additional chemical recovery cycle for sulfur and/or methanol. These additional chemical recovery cycles regenerate and produce commercial quality internally purified chemicals, thus meeting a high quality demand set for, for example, chlorine dioxide plant feedstock, methanol, and sulfuric acid. The introduction of chemicals from outside the process can be reduced, reducing the amount of surplus sulfur in the pulp mill, the cost incurred in purchased chemicals, and the use of fossil-based chemicals.

The new process optimizes the chemical balance in a sulfate pulp mill that produces at least chlorine dioxide bleached pulp and has a chlorine dioxide plant that uses at least chlorate, methanol and sulfuric acid to produce chlorine dioxide. Done and rectified. The method comprises the following steps:
a) gas from a plant's concentrated non-condensable gas (CNCG) system is incinerated to form a gas containing sulfur dioxide ( _SO2 ) and processed to produce concentrated sulfuric acid; and b) a plant process. c) a side stream containing sodium and/or sulfur compounds produced in the factory process is used as make-up chemical;
including at least
Moreover, the production of chlorine dioxide uses the sulfuric acid produced in step a) and methanol purified in step b) such that the sulfuric acid concentration is 94-99%, preferably 95-98%, and In step c) the sodium sulphate or sesquisulphate produced during the production of chlorine dioxide is used.

FIG. 1 presents diagrammatically one embodiment for the implementation of the new method of the invention.

Chlorine dioxide is produced from sodium chlorate, sulfuric acid and methanol in a manner known per se, for example using the processes under the product designations R8 or R10. Such processes are described, for example, in US Pat. Nos. 4,081,520 and 5,066,477. In the new process, the methanol and sulfuric acid used to produce chlorine dioxide are produced internally at the pulp mill by producing commercial grade methanol and sulfuric acid in a new chemical recovery cycle.

In one embodiment of the new process, the sulfuric acid produced in step a) is used in the following processes: tall oil production, lignin recovery from black liquor, pulp bleaching, pH adjustment in mill chemical recovery processes, microcrystalline cellulose (MCC) ) is also used in at least one of Sulfuric acid is also required for certain methanol purification processes, for which internally produced acid is available. In one preferred embodiment, step b) uses the sulfuric acid produced in step a) if the raw methanol is acidified in the methanol refining process. Using internally produced sulfuric acid reduces the input of externally supplied sulfur into the chemical recovery cycle. This also reduces the need to discard large amounts of recovery boiler ash from the chemical recovery cycle.

In one embodiment of the process, step c) comprises the following sidestreams: ash from recovery boilers, sesquisulphate or sodium sulphate from chlorine dioxide production, waste acid from chlorine dioxide plants, EOP from pulp bleaching plants At least one alkaline filtrate, such as the bleach stage filtrate, is also used. This stage is the alkaline extraction stage of bleaching using sodium hydroxide, oxygen and peroxides. When internal material streams are used as make-up chemicals, this greatly reduces the amount of waste streams containing them from the plant. Additionally, it helps prevent the loss of valuable chemicals that previously had to be discharged as waste to maintain chemical balance.

If ash dumping from the recovery boiler is reduced or perhaps completely stopped, e.g. when the concentration of chlorine and potassium in the black liquor reaches a certain level, the chlorine and potassium contained in the ash will be released into the chemical recovery cycle. This build-up can lead to fouling, plugging and corrosion of the heating surfaces of the recovery boiler. Therefore, it is preferable to reduce the amount of chlorine and potassium in the ash by proper treatment before the ash is recycled into the process. Such treatments include, for example, leaching and crystallization of ash. Ash leaching also uses sulfuric acid.

In one embodiment of the process, the sulfuric acid produced in step a) and the methanol produced in step b) constitute at least 70%, preferably 80% of the need for these chemicals during chlorine dioxide production. do.

In one embodiment of the method, the purified methanol produced in step b) is also used as a nitrogen-free fuel to reduce nitrous oxide (NOx) emissions, thus reducing fossil fuels in pulp mills. and reduce the use of fossil-based chemicals. Methanol produced internally can be used, for example, as fuel in recovery boilers and lime kilns, and as a supplementary fuel in sulfuric acid plants.

In the in-house manufacture of process chemicals in factories, it is important that the chemicals are of commercial quality so that they can replace purchased chemicals. Chlorine dioxide production is one of the processes with stringent requirements on the purity of the input chemicals. However, if the sulfuric acid is produced from the mill's non-condensable gases and the methanol is produced from the raw methanol generated in the mill's pulping process, then the purchased sulfuric acid and fossil-based methanol are products produced in the pulp mill. can be replaced with In that case, the only chemical purchased in chlorine dioxide production is sodium chlorate.

Sulfuric acid is preferably produced by the following method. The concentrated non-condensable gases of the pulp mill are oxidized to sulfur dioxide in the incinerator. The gas is further treated in the presence of oxygen and a catalyst (such as V2O5) to oxidize the sulfur dioxide to sulfur trioxide ( _SO3 ). Sulfur trioxide gas is condensed into water vapor contained in the gas to form sulfuric acid. The sulfuric acid is cooled to storage temperature and excess steam is sent to flue gas treatment. For example, Haldor Topsoe's wet gas sulfuric acid (WSA) process is one such sulfuric acid manufacturing process. Since the acid liquefaction is 100% based on condensation, the final product of the process is concentrated commercial grade sulfuric acid. Thus, concentrated commercial grade sulfuric acid can be produced in-house, greatly reducing the amount and cost of purchased chemicals. The concentration of sulfuric acid is 94-99%, preferably 95-98%.

Because the sulfuric acid production process operates above the dew point of sulfuric acid, from incineration all the way to the condenser, metal hardware can be constructed from standard materials such as carbon steel without the problem of corrosion within the system.

Sulfuric acid production at the plant reduces the amount of fly ash dumping corresponding to the reduction in surplus sulfur. In most cases, excess sulfur emissions from the process can be eliminated entirely, which also helps avoid large sodium losses. This, in turn, reduces the amount of make-up sodium hydroxide required and associated costs.

Methanol is obtained as a by-product during the sulfate process in pulp mills. The raw methanol obtained contains impurities such as sulfur compounds, ethanol, ammonium and turpentine. Raw methanol is obtained primarily from the condensate of black liquor evaporation. It is also obtained from other pulp mill items such as cooking stage steam and condensate.

Sulfur compounds and other impurities can be purified from raw methanol to produce commercial grade methanol, for example using the process presented in publication WO/2015/053704. The methanol content of raw methanol is usually at least 65% by weight. In such cases, the raw methanol is washed with a non-polar organic solvent, the treatment can be carried out at the pulp mill and the purified methanol can be used in the mill process. Solvents can be recycled and reused. Non-polar organic solvents are typically solvents containing at least 50% by weight of C8-C20 alkanes and/or cycloalkanes or containing at least 50% by weight of triglycerides that dissolve at temperatures below 40°C. Solvents can be, for example, white mineral oil, mineral oil, white oil, paraffin oil or mixtures thereof. Prior to solvent treatment, raw methanol is usually acidified with sulfuric acid and the resulting precipitate, mainly ammonium sulfate ((NH) ₂ SO ₄ ), is extracted from methanol. Ammonium sulfate is available in biological wastewater treatment plants. The turpentine oil can also be vented to distill off sulfur impurities prior to solvent treatment. Pretreated raw methanol is typically diluted with water prior to solvent treatment.

Alternatively to the methanol purification process described above, other processes can be used to produce pure methanol.

Commercial grade methanol must meet the quality requirements set by the IMPCA (International Methanol Producers & Consumers Association, IMPCA Reference Specification). That is, methanol is essentially free of sulfur and nitrogen.

In the new process, recycling of methanol and sulfur forms an additional chemical recovery cycle. A sulfuric acid plant is an additional chemical recovery cycle that recovers sulfur in concentrated non-condensable gases and recycles it to sulfuric acid. This creates a new kind of sulfur cycle in relation to existing chemical recovery cycles. Methanol is recovered, for example, from plant condensate and used in purified form for the production of chlorine dioxide. This forms a new type of methanol cycle. This sulfur cycle and methanol cycle form a new type of chemical recovery cycle associated with the production of chlorine dioxide.

The new method is described in more detail with reference to the provided figures, which schematically present one embodiment for its implementation.

Sulfuric acid plant 14 produces sulfuric acid from factory concentrated non-condensable gas (CNCG) 2 . This is oxidized to sulfur dioxide in an incinerator. Incineration requires 1 support fuel. This may be methanol and oxygen. The energy produced in the incinerator is recovered as steam 7, usually medium pressure steam (10-45 bar). After incineration, the gas is further treated in the presence _of oxygen and a catalyst (such as _V2O5 ) to oxidize the sulfur dioxide to sulfur trioxide. Sulfur trioxide gas is condensed into water vapor contained in the gas to form sulfuric acid. The sulfuric acid is cooled to storage temperature and excess steam is sent to flue gas treatment. Since acid liquefaction is 100% based on condensation, the final product of this process is concentrated commercial grade sulfuric acid. The concentration of sulfuric acid is 94-99%, preferably 95-98%.

The sulfuric acid produced can be used according to the new process for chlorine dioxide production at plant 15 and the sulfuric acid is transported via line 18 .

The raw methanol produced at the pulp mill is refined in a process that produces the clean methanol needed at the chlorine dioxide plant. This process includes the following stages at plant 16 . Raw methanol 4 containing sulfur and ammonium compounds is oxidized with sulfuric acid 5 to form a precipitate containing ammonium sulfate 13 . It can be used in biological wastewater treatment plants as a nutrient source for microorganisms. Sulfuric acid is preferably factory-produced as described above and conveyed from sulfuric acid plant 14 through line 17 . During acidification, turpentine 12 can be extracted from raw methanol by decantation. After acidification and removal of precipitate, methanol is distilled and the distilled methanol is diluted with 6 parts of water. The turpentine can be drained from the diluted methanol before washing the methanol with a non-polar organic solvent. Methanol, ethanol, and acetone can also be removed from the washed methanol by distillation. This allows for the production of clean methanol 11 required for the production of chlorine dioxide from in-plant sources.

Sulfuric acid gas separated during methanol refining is sent via line 19 to sulfuric acid production.

Sulfuric acid and methanol produced in-house are used in chlorine dioxide plant 15 with sulfuric acid being conveyed via line 18 and methanol via line 20 . Chlorine dioxide 9 is prepared in a manner known per se from sodium chlorate 3, for example using process R8 or R10. Chlorine dioxide plants produce by-products containing sulfur and sodium 10. The R8 process also produces the acid salt, sodium sesquisulfate 10, and the R10 process produces sodium sulfate 10. It can be brought into the chemical recovery cycle as a make-up chemical.

Sulfuric acid 8 can also be utilized in the mill's potential tall oil plant, for example for recovery of lignin from black liquor, bleaching of pulp. The purified methanol 11 can also be used as a supplementary fuel 1 for sulfuric acid plants and as fuel for recovery boilers and/or lime kilns.

Claims (6)

1. A method of controlling chemical balance in a sulfate pulp mill having at least chlorine dioxide bleached pulp and having a chlorine dioxide plant using at least chlorate, methanol and sulfuric acid to produce chlorine dioxide, the method comprising: The following steps:
a) the gas from the plant's concentrated non-condensable gas system is incinerated to form a gas containing sulfur dioxide and processed to produce concentrated sulfuric acid; and b) the raw methanol from the plant process is purified. and c) a sidestream containing sodium and/or sulfur compounds produced in the factory process is used as a make-up chemical.
including at least
Moreover, the production of chlorine dioxide uses the sulfuric acid produced in step a) and methanol purified in step b) such that the sulfuric acid concentration is 94-99%, preferably 95-98%, and A process as described above, wherein in step c) sodium sulphate or sesquisulphate produced during the production of chlorine dioxide is used. The sulfuric acid produced in step a) is also used in at least one of the following processes: tall oil production, lignin recovery from black liquor, pulp bleaching, pH adjustment in mill chemical recovery processes, production of microcrystalline cellulose. A method according to claim 1, characterized in that: Step c) removes at least one of the following sidestreams: ash from a recovery boiler, sesquisulphate or sodium sulphate from chlorine dioxide production, waste acid from a chlorine dioxide plant, alkaline filtrate from a pulp bleaching plant. 3. A method according to claim 1 or 2, characterized in that it also uses Claim characterized in that the sulfuric acid produced in step a) and the methanol produced in step b) constitute at least 70%, preferably 80% of the need for these chemicals during chlorine dioxide production. 4. The method according to Item 1, 2 or 3. 5. The method according to any one of claims 1 to 4, characterized in that the purified methanol produced in step b) is also used as a nitrogen-free fuel in order to reduce nitrous oxide emissions. Method. In step a), a gas containing sulfur dioxide is added in the presence of oxygen and a catalyst to oxidize the sulfur dioxide to sulfur trioxide, which is condensed into the water vapor contained in the gas to produce concentrated sulfuric acid. A method according to any one of claims 1 to 5, characterized in that it is treated under.

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